Liquid crystal screen display

ABSTRACT

A liquid crystal screen display is disclosed which is capable of restraining occurrence of display unevenness. In the display of the invention, a conductive member to which a negative voltage is applied is placed between a substrate and an alignment layer so as to be in partial contact with the alignment layer. Uneven ion distribution attributable to ion generation within broken or pin hole parts of an overcoat film is restrained, for instance, by exposing gate signal lines etc. to the alignment layer so that ions are intentionally generated in the regions of the liquid crystal layer corresponding to the exposed regions.

TECHNICAL FIELD

[0001] The present invention relates to a liquid crystal screen displayand more particularly to an improved method for restraining occurrenceof display unevenness in active matrix liquid crystal screen displays.

BACKGROUND OF THE INVENTION

[0002] Liquid crystal screen displays possess advantages such asthinness, light weight, and applicability to low-voltage driving, andtherefore, they have been used in many display applications includingwrist watches, desk-top calculators, personal computers and wordprocessors as well as in light shutter applications.

[0003] The most popular liquid crystal screen displays are the TwistedNematic (TN) mode liquid crystal screen displays in which a liquidcrystal layer is held between a pair of substrates and electrodes areplaced on the respective substrates, for establishing an electric fieldfor driving liquid crystal molecules in the liquid crystal layer. The TNmode liquid crystal screen displays do not have a sufficiently wideangle of view. Up to now, there have been proposed various driving modeliquid crystal screen displays having wider angles of view than the TNmode liquid crystal screen displays, these displays including the IPS(In-Plane Switching) mode in which a pair of electrodes for drivingliquid crystal molecules are placed within the same pixel on the samesubstrate; the PVA (Patterned Vertical Alignment) mode in which a pairof substrates are alternately provided with a substantially linearelectrode within the same pixel; the OCB mode (Optically CompensatedBirefringence) mode which has an electrode layout similar to that of theTN mode but provides wider angles of view; and the MVA (Multi-domainVertical Alignment) mode.

[0004]FIG. 17 shows one example of array substrates for use in the IPSmode liquid crystal screen displays. FIGS. 18a, 18 b and 18 c showsectional views of a liquid crystal screen display that uses thesubstrate shown in FIG. 17. In a pixel region defined by a pair of gatesignal lines 4 and a pair of source signal lines 5 on an array substrate2, pixel electrodes 6 and common electrodes 7 are both laid out in acomb-like fashion and placed with an insulating layer 11 between. Thecommon electrodes 7 within one pixel region are electrically connectedto those of the adjacent right and left pixel regions (in the drawing)by a common electrode line 8 integrally formed therewith. The sourcesignal lines 5 formed on the same layer as the pixel electrodes 6 areconnected to the pixel electrodes 6 through a thin film transistor (TFT)9. The gate signal lines 4 are formed on the same layer as the commonelectrodes 7 and others to supply the TFT 9 with a signal forcontrolling the electrical connection between the source signal lines 5and the pixel electrodes 6. In the region where the pixel electrodes 6and the common electrode line 8 are overlapped, a storage capacitor 23is formed.

[0005] An overcoat film (passivation film) 12 is laid over the surfaceof the array substrate 2 having these signal lines and electrodes formedthereon, and an alignment layer 16 is further formed so as to cover theovercoat film 12.

[0006] A liquid crystal screen display 1 comprises the array substrate 2shown in FIG. 17 and an opposed substrate 3 which faces the arraysubstrate 2 across a liquid crystal layer 18. On the surface of theopposed substrate 3 facing the array substrate 2, a lattice-like blackmatrix 14 for defining the pixel regions and a color filter 15 havingsections corresponding to the pixel regions are formed and the alignmentlayer 16 is formed so as to cover them. Since the region immediatelyabove the common electrode line 8 does not contribute to normaldisplaying by the pixels, a black matrix is sometimes used, in place ofthe color filter, in the region where the opposed substrate 3 faces thecommon electrode line 8.

[0007] In the formation of electrodes and signal lines on a substratefor liquid crystal screen displays, these elements are liable toelectrical short owing to contamination with dust particles or patterndefects of the electrodes. Above all, array substrates for IPS modeliquid crystal screen displays are most likely to cause electrical shortin their formation process because of the provision of comb-like pixelelectrodes and common electrodes.

[0008] If the short-circuited portions are isolated by laserirradiation, the portions of the overcoat film which have beenirradiated with the laser beams tend to be destroyed so that the signallines and others reveal from the overcoat film. If the gate signal linesare exposed resulting from the laser irradiation, display unevenness islikely to occur in the regions repaired by the laser irradiation, whenthe liquid crystal screen display having such a substrate iscontinuously driven. In the case of a normally black display panel forinstance, images displayed by the pixels in repaired regions are darkerthan those displayed by the surrounding pixels.

[0009] In fact, an IPS mode liquid crystal screen display was fabricatedby use of an array substrate in which an overcoat film in the regionsprovided with gate signal lines had been removed by laser irradiationand was continuously driven at a temperature of 50° C. for twelve hoursin a thermostatic oven. When images of intermediate gray scale aredisplayed by this display, display unevenness was observed in theregions which had been subjected to laser irradiation. This displayunevenness is conceivably attributable to the fact that the voltageretention of the liquid crystal layer in the laser-irradiated regionslocally decreases owing to local ion generation in the regions of theliquid crystal layer surrounding the laser-irradiated regions or toimpurity ions which have been included in the liquid crystal layerbeforehand and adsorbed in the laser-irradiated regions. In the regionswhere the overcoat film is destroyed, the signal lines having aspecified potential are exposed. Therefore, electrons are introducedinto the liquid crystal layer from the signal lines or the like andliquid crystal molecules are decomposed or electrically charged,resulting in ion generation or ion adsorption in the regions. The localuneven distribution of ions causes a drop in the voltage retention ofthe liquid crystal layer of these regions and, in consequence, displayunevenness. Since the so-called reverse driving for alternatelyreversing the polarities of the pixel electrodes relative to the commonelectrodes is generally adopted, exposed elements generate ions in acertain period and retrieve ions in another period. Accordingly, therearises no serious problems even if the overcoat film in the regionswhere the pixel electrodes, the common electrodes and the source signalline are formed is destroyed. In contrast with this, if the overcoatfilm is destroyed in the regions where gate signal lines are formedwhose polarity is negative with respect to the pixel electrodes and thecommon electrodes, negative ions are substantially constantly generatedor positive ions are adsorbed in these regions. Therefore, ionconcentration becomes extremely high in these regions, compared to otherregions. Similar display unevenness occurs, for example, in cases wherepin holes are created in the overcoat film laid over the gate signallines or where the gate signal lines have level differences.

[0010] To restrict the influence of pinholes, Japanese PatentPublication (KOKAI) No. 10-206857, for example, proposes that theovercoat film be 0.4 μm or more thicker than the electrodes in contactwith the overcoat film. According to this publication, although theexposure of the electrodes to the liquid crystal layer through pin holescan be reduced by use of an overcoat film having sufficient thickness,it has no effect of restraining the exposure of signal lines and othersdue to the destruction of the overcoat film by laser irradiation.

[0011] Japanese Patent Publication (KOKAI) No. 10-186391 has proposedthat part of the electrodes be formed in contact with an alignment layerand the resistivity of the liquid crystal material be 10¹³Ω or more inorder to restrain display abnormalities due to d.c. components remainingin the overcoat film (i.e., insulating film) of an IPS mode liquidcrystal screen display. By exposing the pixel electrodes, ions in theliquid crystal layer can be retrieved on the exposed surface. However,since the voltage of the pixel electrodes is retained only by thestorage capacitor during most of the period of time when the panel isdriven, active retrieval of ions results in a drop in interelectrodevoltage. More concretely, the ion concentration of the liquid crystallayer cannot be effectively reduced. Selection of a liquid crystalmaterial based on resistivity is not necessarily appropriate in view ofprevention of image persistence and high-speed response.

[0012] In such a situation, there have been demands toward liquidcrystal screen displays capable of effectively restraining occurrence ofdisplay unevenness attributable to ions which have been locallygenerated within the liquid crystal layer.

DISCLOSURE OF THE INVENTION

[0013] A primary object of the invention is to provide a liquid crystalscreen display capable of restraining display unevenness attributable toions which have been locally generated within the liquid crystal layerand displaying satisfactory images for a long time.

[0014] According to the present invention, there is provided a liquidcrystal screen display comprising: a first insulating substrate; asecond insulating substrate facing the first insulating substrate; aliquid crystal layer formed between the first and second insulatingsubstrates; alignment layers formed between the first insulatingsubstrate and the liquid crystal layer and between the second insulatingsubstrate and the liquid crystal layer, respectively, for aligning theliquid crystal layer; and a first conductive member which is formedbetween at least either one of the first and second insulatingsubstrates and its corresponding alignment layer, being in partialcontact with the alignment layer and to which a negative voltage isapplied, the liquid crystal screen display further comprising anotherconductive member which is formed between at least either one of thefirst and second insulating substrates and its corresponding alignmentlayer, being in partial contact with the alignment layer, and to which anegative voltage is applied.

[0015] An overcoat film is provided with apertures in the regions, wherea conductive member having a stable polarity that is substantiallypositive or negative with respect to the pixel electrodes and the commonelectrodes is placed, such that part of the conductive member is incontact with the liquid crystal layer or faces the liquid crystal layeracross only an alignment layer. The conductive member generates ions inthe regions where the apertures are formed, like ion generation in laserirradiated regions and pin hole regions. Specifically, the conductivemember, examples of which include signal lines and electrodes, is madeto be in contact with the liquid crystal layer or opposed to the liquidcrystal layer through only the alignment layer, thereby intentionallyforming regions where ions are generated. The display unevenness of theliquid crystal screen displays described earlier is caused by formationof local regions within the liquid crystal layer, the regions having ionconcentrations different from that of other regions. It means that ifthe ion concentration of the liquid crystal layer is uniformirrespective of its degree, display unevenness will not occur.

[0016] For effective ion generation in the liquid crystal layer, it isdesirable to use gate signal lines as the above conductive member. Thegate signal is in its ON state only for a period of one horizontal scanduring the flame scan period and in its OFF state in other periods.Accordingly, the polarity of the gate signal lines relative to the pixelelectrodes and the counter electrodes is negative for most of the flamescan period. Ion generation is likely to occur mainly in negativeelectrodes and particularly in the gate signal lines. The potential ofthe counter electrodes is constant or has insignificant fluctuations sothat it does not contribute to ion generation. Generally, the polarityof the source signal lines is alternately reversed relative to thecounter electrodes, so that the source signal lines generate ions andretrieve them. The conductive member may be a third electrode that isdriven independently of the pixel electrodes and the common electrodes.It, however, should be noted that the conductive member may be othersignal lines and electrodes, depending on the form of drive signals.

[0017] Preferably, the percentage of area of the conductive materialexposed to the alignment layer ranges from 10% to 50% in order to attainuniform ion concentration for the liquid crystal layer and to avoid anexcessive removal of the overcoat film 12.

[0018] Occurrence of electrical short can be more effectively restrainedby forming an insulating film on the surfaces of the gate signal linesby anodic oxidation in the regions where the gate signal lines intersectother signal lines. For instance, the regions above which an aperture isto be formed and the regions (e.g., connector terminals) where theconductivity of the surfaces of signal lines needs to be ensured aremasked while an anodic oxidation film being selectively formed in otherregions. As the material of the gate signal lines, aluminum, tantalum,zirconium and alloys containing these metals in an amount of 80 wt % ormore are suitably used in the light of easy anodic oxidation and thestability of an oxide film to be formed.

[0019] As the ion concentration of the liquid crystal layer increases,the resistivity of the liquid crystal layer decreases, causingfluctuations in the substantial driving voltage applied to the liquidcrystal layer. It is therefore preferable to further provide aconductive member for retrieval of ions from the liquid crystal layer inorder to maintain the ion concentration of the liquid crystal layer at alow level.

[0020] Like the conductive member for ion generation, the conductivemember for ion retrieval is placed such that at least part of it is incontact with the liquid crystal layer or disposed adjacent the liquidcrystal layer with the alignment layer between, and has specifiedpotential. The conductive member has a polarity that is positive withrespect to the gate signal lines. For example, the pixel electrodes orcommon electrodes may be used as the conductive member. Although greateffects cannot be expected by using these electrodes for ion retrievalin a liquid crystal screen display having no ion generation means, theion concentration of the liquid crystal layer can be maintained within aspecified range by retrieving ions with the conductive member whilegenerating ions. Examples of the conductive member further include othersignal lines and the third electrode.

[0021] In general, the pixel electrodes and the common electrodes areformed on different layers with an insulating layer between. Forexposing the electrodes on the lower layer to the alignment layer or tothe liquid crystal layer, such a process is taken that electrodecomponents on the same layer as the upper electrodes or on the furtherupper layer are added to the electrodes on the lower layer and then, anaperture is formed in the overcoat film laid over the electrodecomponents thus added.

[0022] When forming the lower electrodes and the gate signal lines atthe same time, a sufficient margin can be obtained between the gatesignal lines and the electrodes formed on the same layer as the gatesignal lines by providing electrode components for a region adjacent thegate signal lines. In this way, occurrence of electrical short at thetime of formation of these elements can be restrained. Electrodecomponents are positioned in the vicinity of the gate signal lines butformed on a layer different from the layer on which the gate signallines are formed, with an insulating layer being interposed betweenthese layers.

[0023] In addition, the signal lines or the electrodes on the upperlayer can be easily exposed by providing an overcoat film for only theregion including a switching element while no overcoat film is laid overthe electrodes and the signal lines.

[0024] A substance having ion retrievablility is placed so as to be incontact with the liquid crystal layer or the alignment layer. Examplesof such a substance include adsorptive substances such as aluminumoxide; physical adsorptive substances such as porous glass and poroussilicon that adsorb ions; and chemical adsorptive substances such as ionexchange resin.

[0025] The size of the apertures formed in the overcoat film for iongeneration and ion retrieval affects the ion generation capability andion retrievablility of the apertures. For obtaining a means havingsatisfactory ion generation capability and ion retrievablility, thediameter or side length of the apertures is preferably 5 μm or more. Torestrain the influence of the apertures upon display quality, thediameter or side length of the apertures should be no more than 100 μmwhich is difficult to be seen by the naked eye.

[0026] To avoid the influence of the panel upon display quality, thethird electrode serving as the conductive member for ion generation orion retrieval is preferably placed on the black matrix of the opposedsubstrate or in the region opposed to the common electrode line. Theseregions are not associated with normal displaying by the pixels in aplan view of the display device.

[0027] Provision of a conductive member for ion generation for everypixel most effectively restrains occurrence of display unevenness. It isapparent that a conductive member may be provided for every specifiednumber of pixels. The distribution of the conductive members isdetermined taking into consideration their ion generation capability aswell as the influence of their arrangement upon the display quality ofthe panel. The distribution of the conductive members for ion retrievalis also determined by taking account of their ion retrievablility andthe influence of their arrangement upon the display quality of thepanel. Since the visibility of humans to luminance variations in blue islower than to luminance variations in other colors (i.e., red andgreen), provision of the conductive members for blue pixels reduces afeeling of physical disorder given to human beings, compared to the casewhere other color pixels are provided with the conductive members.

[0028] In another liquid crystal screen display of the invention,electrolytic salt is added for forming ions within the liquid crystallayer. Where electrolytic salt is added to the liquid crystal layerbeforehand, even if ions are locally generated, the difference in ionconcentration between regions and, in consequence, display unevennessoccurring in the panel are reduced.

[0029] One preferable example of the electrolytic salt is a chemicalcompound represented by the chemical formula: (t-Bu)₄NX. Preferably, Xis halogen or COOR (herein, R is hydrogen, a hydro carbon group oralkali metal).

[0030] The invention is applicable to all the driving modes of liquidcrystal displays. It is particularly useful for the IPS mode and PVAmode liquid crystal screen displays which require elaborate processingfor the formation of the electrodes and others.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 is a partially cut-away plan view showing an essential partof an array substrate for use in a liquid crystal screen displayaccording to one embodiment of the present invention.

[0032]FIG. 2 is a longitudinal sectional view showing the essential partof the liquid crystal screen display.

[0033]FIG. 3 is a plan view showing an essential part of an arraysubstrate having a pattern abnormality, the substrate being prepared forevaluation in the above embodiment.

[0034]FIG. 4 is a longitudinal sectional view showing an essential partof a liquid crystal screen display according to another embodiment ofthe present invention.

[0035]FIG. 5 is a longitudinal sectional view showing an essential partof a liquid crystal screen display according to still another embodimentof the present invention.

[0036]FIG. 6 is a partially cut-away plan view showing an essential partof an array substrate for use in a liquid crystal screen displayaccording to still another embodiment of the present invention.

[0037]FIG. 7a is a sectional view taken along line B-B′ of FIG. 6, andFIG. 7b is a sectional view taken along line C-C′ of FIG. 6.

[0038]FIG. 8 is a partially cut-away plan view showing an essential partof an array substrate for use in a liquid crystal screen displayaccording to still another embodiment of the present invention.

[0039]FIG. 9a is a sectional view taken along line D-D′ of FIG. 8, FIG.9b is a sectional view taken along line E-E′ of FIG. 8, and FIG. 9c is asectional view taken along line F-F′ of FIG. 8.

[0040]FIG. 10 is a partially cut-away plan view showing an essentialpart of an array substrate for use in a liquid crystal screen displayaccording to still another embodiment of the present invention.

[0041]FIG. 11a is a sectional view taken along line G-G′ of FIG. 10, andFIG. 11b is a sectional view taken along line H-H′ of FIG. 10.

[0042]FIG. 12 is a partially cut-away plan view showing an essentialpart of an array substrate for use in a liquid crystal screen displayaccording to still another embodiment of the present invention.

[0043]FIG. 13a is a sectional view taken along line I-I′ of FIG. 12, andFIG. 13b is a sectional view taken along line J-J′ of FIG. 12.

[0044]FIG. 14 is a partially cut-away plan view showing an essentialpart of an array substrate for use in a liquid crystal screen displayaccording to still another embodiment of the present invention.

[0045]FIG. 15 is a partially cut-away plan view showing an essentialpart of an array substrate for use in a liquid crystal screen displayaccording to still another embodiment of the present invention.

[0046]FIG. 16 is a longitudinal sectional view showing an essential partof a liquid crystal screen display according to still another embodimentof the present invention.

[0047]FIG. 17 is a partially cut-away plan view showing an essentialpart of an array substrate for use in a conventional liquid crystalscreen display.

[0048]FIG. 18a is a sectional view taken along line K-K′ of FIG. 17,FIG. 18b is a sectional view taken along line L-L′ of FIG. 17, and FIG.18c is a sectional view taken along line M-M′ of FIG. 17.

BEST MODE FOR CARRYING OUT THE INVENTION

[0049] With reference to the accompanying drawings, preferredembodiments of the present invention will be hereinafter described indetails, taking IPS mode liquid crystal screen displays for example.

First Embodiment

[0050]FIG. 1 shows an array substrate for use in a liquid crystal screendisplay of the first embodiment. The liquid crystal screen displayemploying this array substrate is shown in FIG. 2.

[0051] The array substrate 2 has electrodes and signal lines in asimilar layout to that of the array substrate of the prior art shown inFIG. 17. In a pixel region enclosed by a pair of gate signal lines 4 anda pair of source signal lines 5, pixel electrodes 6 and commonelectrodes 7, which are both comb-like in shape, are placed with aninsulating layer 11 between. The common electrodes 7 in one pixel regionare electrically connected to common electrodes 7 within the right andleft adjacent pixel regions shown in FIG. 1 through a common electrodeline 8 that is integrally formed with the common electrodes 7. Thesource signal lines 5 formed on the same layer as the pixel electrodes 6are connected to the pixel electrodes 6 through a thin film transistor(TFT) 9. The gate signal lines 4 are formed on the same layer as thecommon electrodes 7 and others, for supplying a signal to the TFT 9 tocontrol the electrical connection between the source signal lines 5 andthe pixel electrodes 6.

[0052] An overcoat film 12 is laid over the surface of the arraysubstrate 2 on which these signal lines and electrodes are formed. Analignment layer 16 is formed so as to cover the overcoat film 12.

[0053] A black matrix 14 of a lattice shape for defining pixel regionsand a color filter (not shown) having sections corresponding to therespective pixel regions are formed on the surface of an opposedsubstrate 3 that faces the array substrate 2 across a liquid crystallayer 18. Another alignment layer 16 is further disposed so as to coverthem.

[0054] The array substrate 2 has overcoat film apertures 13 on theovercoat film 12 immediately above the gate signal lines 4, as shown inFIGS. 1 and 2. In the region where each aperture 13 is formed, a gatesignal line 4 is in contact with an alignment layer 16 as shown in FIG.2. The gate signal lines 4 in the regions where they are exposed to thisalignment layer generate ions within the liquid crystal layer 18 whenthe panel is in service.

[0055] The array substrate 2 is fabricated, for example, in thefollowing way.

[0056] A conductive film is formed from aluminum etc. on the surface ofthe glass substrate 10, and then, this conductive film is processed toform the gate signal lines 4, the common electrodes 7, and the commonelectrode line 8. Regarding the size of the apertures 13, the area ofeach aperture 13 in a plan view accounts for 10% to 50% of each gatesignal line 4. If the area of the aperture 13 is less than 10%, the sizeof the aperture 13 is not sufficient to attain uniform ion concentrationin the liquid crystal layer without difficulty. If the area of theaperture 13 exceeds 50% on the other hands, the amount of removal of theovercoat film 12 will increase excessively. It is preferable that thearea accounting for 15% to 40% of each gate signal line 4 be opened byeach aperture 13.

[0057] Then, an insulating film is formed from silicon oxide or the likeso as to cover them and a semiconductor layer for the TFT 9 having aspecified shape is further formed on the insulating film. The insulatingfilm formed herein also serves as a gate insulating film for the TFT 9.After filling a specified region of the semiconductor layer thus formedwith impurities, an insulating layer is similarly formed from siliconoxide etc. so as to cover them. The two layers of insulating filmsconstitute the insulating film 11.

[0058] After the formation of the insulating film 11, a contact hole isformed at a specified position within the region where the semiconductorlayer of the substrate 10 is formed. An aluminum film and a titaniumfilm are formed to cover the entire surface of the substrate 10 and thismultiple layered film is processed to form the source signal lines 5integral with the source electrode of the TFT 9 and the pixel electrodes7* integral with the drain electrode of the TFT 9.

[0059] In this way, the overcoat film 12, which is made from siliconnitride or the like and covers the surface of the substrate 10 havingthe signal lines and the electrodes thereon, is formed, and theapertures 13 are formed by selectively removing the overcoat film 12 atspecified positions within the regions where the gate signal lines 4 areformed. At the same time, the overcoat film 12 formed on the peripheraledge of the substrate 10 is removed as necessary, to expose a connectorterminal (not shown) that is placed for connection between the signallines formed there and an external driving circuit. Thereafter, thealignment layer 16 made from polyimide or the like is formed on thesurface of the substrate 10 in a specified manner and the arraysubstrate having the apertures 13 (such as shown in FIG. 1) at whichpart of the respective gate signal lines 4 is in contact with thealignment layer 13* is obtained.

[0060] Where the gate signal lines 4 are exposed by removing theovercoat film and the alignment layer with a laser such as described inBackground Art, electrons also move from the gate signal lines 4 to theliquid crystal layer 18 so that liquid crystal molecules are ionized.However, the ionized liquid crystal molecules, in this case, are likelyto be accumulated at the exposed portions of the gate signal lines 4,because the exposed portions of the gate signal lines 4 are small. Theaccumulation of the ionized liquid crystal molecules at the exposedportions of the gate signal lines 4 causes variations in ionconcentration, which can be the cause of display unevenness. In the caseof the present embodiment, electrons move from the gate signal lines 4to the liquid crystal layer 18 through an extremely thin alignment layer13*, the gate signal lines 4 being supplied with a negative voltage atall times except when the TFT 9 is in its ON state. Therefore, liquidcrystal molecules are ionized (i.e., an ionization) and the ionizedliquid crystal molecules extensively disperse throughout the liquidcrystal layer 18 so that the ion concentration of the liquid crystallayer 18 becomes uniform as a whole (i.e., variations in ionconcentration are eliminated). This is thought to be the reason whydisplay unevenness does not occur in the present embodiment.

[0061] In some cases, excessively high ion concentration resulting froman excessive number of liquid crystal molecules which have been ionizedwithin the liquid crystal layer 18 causes troubles in displaying. It isconceivable that, in the present embodiment, ionized liquid crystalmolecules extensively disperse within the liquid crystal layer 18 sothat the ion concentration of the liquid crystal layer 18 does notincrease, on the whole, to the extent that troubles are caused indisplaying.

[0062] A test was actually conducted in which after a patternabnormality portion 20 was intensively formed between a gate signal line4 and a pixel electrode 6 as shown in FIG. 3, an overcoat film 12 wasformed so as to cover them. The abnormality portion 20 was thenirradiated with a laser beam thereby separating the gate signal line 4and the pixel electrode 6 from each other. Subsequently, apertures 13were formed in the overcoat film as shown in FIGS. 1 and 2, and analignment layer 16 was further formed to obtain an array substrate 2. Aliquid crystal screen display was fabricated, using this arraysubstrate.

[0063] After the liquid crystal screen display of the present embodimentthus fabricated was continuously driven at 50° C. for 300 hours, itdisplayed good intermediate gray scale images free from unevenness. Inthis experiment, the voltages applied to the source signal lines 5 andapplied to the gate signal lines 4 were set to ±5V (rectangular pulsevoltages) and −10V, respectively.

[0064] The “negative voltage” applied to the first conductive memberrepresented by the gate signal lines 4 means a voltage (e.g., −10V)lower than the lowest voltage (herein, −5V) among the voltages(rectangular pulse voltages of ±5V) applied to the source signal lines 5during the period when the TFT 9 is in the OFF state. In this case, inthe period when the TFT 9 is in the ON state, a voltage of +10V isapplied to the gate signal lines 4.

[0065] In the present specification, the terms “high” and “low” are usedto describe the relationship between two voltages and these terms meanthat an absolute voltage is high or low. For example, +5V is higher than±0V, ±0V is higher than −5V, and −5V is higher than −10V.

[0066] In the case of the conventional liquid crystal screen displayhaving no overcoat film aperture, unevenness was found in the portionsrepaired by laser irradiation after continuous driving in the samecondition.

[0067] In contrast with the conventional fabrication method, the presentembodiment does not require an additional process. More specifically, inthe step of exposing the connector terminal, which is used forconnection between the signal lines and the external driving circuit,from the overcoat film 12, the apertures 13 may be formed at the sametime. The liquid crystal screen display of the present embodimentcapable of restraining occurrence of display unevenness can be attainedonly by changing the configuration of the mask used in the step ofprocessing the overcoat film 13* in the conventional fabricationprocess.

[0068] The insulation of the elements with respect to one another can beenhanced by formation of an insulating film by anodic oxidation on thesurfaces of the signal lines in the regions including the crossing partsof the signal lines and the gate electrode of the TFT 9, so thatoccurrence of electrical short between them can be effectivelyrestricted. For instance, after formation of the gate signal lines 4containing aluminum, an anodic oxidation film 4 a is formed fromaluminum oxide on the surfaces of the gate signal lines 4 in specifiedregions as shown in FIG. 4, by anodic oxidation by use of a mask whichhas a pattern for covering the regions of the apertures 13 and theconnector terminal for connection between the signal lines and theexternal driving circuit.

Second Embodiment

[0069] While the first embodiment has been presented in terms of gatesignal lines used as a member for ion generation, the second embodimentis associated with another form.

[0070] A third electrode, which is formed in contact with the alignmentlayer independently of the pixel electrodes and the common electrodes,is provided for the conventional liquid crystal screen display as aconductive member for ion generation, whereby a liquid crystal screendisplay capable of restraining occurrence of display unevenness due touneven ion distribution can be obtained, similarly to the firstembodiment. The liquid crystal screen display according to the secondembodiment comprises the array substrate 2 having a structure similar tothat of the conventional liquid crystal screen display shown in FIG. 17and the opposed substrate 3 which includes a third electrode 17 having anegative potential with respect to the electrodes 6 and 7, as shown inFIG. 5.

[0071] The third electrode 17 is in contact with the alignment layer 18*and its potential is set equal to the OFF potential (i.e., the potentialof the gate signal lines 4 when the TFT 9 connected to it is in the OFFstate). For instance, the potential of the pixel electrodes 6 isreversed at ±5V with respect to the common electrodes 7 when the panelis in service and the common electrodes 7 are grounded (0V). While thepanel is driven, the third electrode 17 is maintained at −10V and minusions are constantly generated.

[0072] The third electrode is placed in a region opposed to a gatesignal line of the opposed substrate, or alternatively placed in such aregion on the array substrate or on the opposed substrate that theplacement of the third electrode does not significantly affect displayquality. Examples of such a region are the regions where a storagecapacitor is disposed and where a source signal line is placed.Preferably, the third electrode is overlapped with the black matrix 14on the opposed substrate 3. The black matrix 14 made from a conductivematerial such as chrome may be used as the third electrode.

Third Embodiment

[0073] If ions are continuously generated like the foregoingembodiments, the resistivity of the liquid crystal layer graduallydecreases, restraining display unevenness, while there is thepossibility of deterioration in display quality due to a constantdecline in voltage retention during long use. Although it is possible tocompensate for such changes by use of drive signals, the thirdembodiment will be described in terms of one example of liquid crystalscreen displays capable of fundamentally preventing deterioration indisplay quality without compensation by use of such drive signals.

[0074]FIG. 6 shows an array substrate for use in the liquid crystalscreen display of the third embodiment. FIGS. 7a and 7 b show the liquidcrystal screen display of the third embodiment.

[0075] The liquid crystal screen display of the third embodiment hasapertures 13 a for ion generation and an aperture 13 b for ionretrieval. The apertures 13 a are formed in parts of the gate signallines by removing the overcoat film 12 therefrom like the firstembodiment. The aperture 13 b is formed in the region of the pixelelectrodes 6 where the storage capacitor is formed. The apertures 13 aand 13 b are formed by processing the overcoat film 12 which has beenformed beforehand, similarly to the formation of the apertures 13 in theforegoing embodiments.

[0076] For most of the driving period, the potential of the pixelelectrodes 6 is higher than that of the gate signal lines 4. Forexample, when the voltage applied to the pixel electrodes 6 is ±5V(rectangular pulse voltage), the potential of the lowest pixel voltageis −5V, but the voltage applied to the gate signal lines 4 during thetime the TFT 9 is in the OFF state is −10V. Therefore, the pixelelectrodes 6 exposed to the liquid crystal layer 18 through the ionretrieval aperture 13 b function to retrieve negative ions within theliquid crystal layer 18. Accordingly, ions are generated by the iongeneration apertures 13 a to make the ion distribution of the liquidcrystal layer 18 uniform, whereas ions are retrieved by the ionretrieval aperture 13 b thereby inhibiting an excessive rise in the ionconcentration of the liquid crystal layer 18. In short, the liquidcrystal screen display of the third embodiment includes a conductivemember for ion generation as well as a conductive member for ionretrieval.

[0077] The liquid crystal screen display thus formed has provedsuccessful in displaying satisfactory images free from displayunevenness after continuous driving at 50° C. for 500 hours.

[0078] In the third embodiment, display unevenness can be eliminated byuniformly distributing ionized liquid crystal molecules within theliquid crystal layer 18 similarly to the first embodiment, whereas theelectrons of the ionized (anionized) liquid crystal molecules are takenaway by the second conductive member represented by the pixel electrodes6 so that the ionized liquid crystal molecules become normal liquidcrystal molecules. With this arrangement, the possibility of troubles indisplaying can be lessened even if the number of ionized liquid crystalmolecules increases too much within the liquid crystal layer 18, leadingto excessively high ion concentration. In addition, the liquid crystalmolecules, which have been anionized upon receipt of electrons at theion generation apertures 13 a from the first conductive memberrepresented by the gate signal lines 4, are attracted by the pixelelectrodes 6 at the ion retrieval aperture 13 b, the pixel electrodes 6having a potential higher than that of the gate signal lines 4. Thiselectric attraction allows the ionized liquid crystal molecules withinthe liquid crystal layer to be quickly dispersed and uniformlydistributed, compared to the first embodiment. This further reducesdisplay unevenness.

[0079] Since the region where the ion retrieval aperture 13 b is formedis covered with the black matrix 14 placed on the opposed substrate 3 asshown in FIG. 7a, the display quality of the panel is not affected bythe provision of the ion retrieval aperture 13 b. Additionally, sincethis region per se does not contribute to normal pixel displaying, itwould give virtually no influence upon the display quality of the paneleven if the liquid crystal screen display had no black matrix 14.

Fourth Embodiment

[0080] In the fourth embodiment, there will be explained an example inwhich an electrode similar to the third electrode of the secondembodiment is used as the ion retrieval conductive member.

[0081] According to the fourth embodiment, there is provided a liquidcrystal screen display incorporating an array substrate having iongeneration apertures in an overcoat film immediately above gate signallines like the first embodiment, and an opposed substrate having a thirdelectrode similar to that of the second embodiment. wherein thepotential of the third electrode is made to be positive relative to thepotential of the gate signal lines. Effective ion retrieval can beperformed by fixing the potential of the third electrode at a specifiedvalue or more preferably at the median value of source signals or avalue proximate to the potential of the common electrodes.

[0082]FIG. 8 shows an array substrate for use in another liquid crystalscreen display according to the fourth embodiment. The array substrate 2is similar to that of the third embodiment, including the ion generationapertures 13 a in the regions where the gate signal lines 4 are disposedand the ion retrieval aperture 13 b positioned immediately above thecommon electrode line 8. As shown in FIGS. 9a, 9 b and 9 c, the opposedsubstrate 3 used for the panel 1 has the third electrode 17 having apotential equal to the potential of the common electrodes. The thirdelectrode 17 is positioned in contact with the liquid crystal layer 16*to retrieve ions.

[0083] This liquid crystal screen display has proved successful indisplaying satisfactory images free from display unevenness aftercontinuous driving at 50° C. for 1,000 hours.

Fifth Embodiment

[0084] The fifth embodiment will be explained in terms of one example inwhich an Ion retrieval aperture is formed in a region of the overcoatfilm immediately above a common electrode.

[0085]FIG. 10 shows an array substrate for use in the liquid crystalscreen display of the fifth embodiment and FIGS. 11a and 11 b show theliquid crystal screen display of the fifth embodiment.

[0086] A common electrode 7 is comprised of a first common electrode 7 aand a second common electrode 7 b, the first common electrode 7 a beingintegrally formed with the gate signal lines 4, the common electrodeline 8 and others while the second common electrode 7 b is located in alayer upper than those elements with the insulating layer 11 between.The first common electrode 7 a and the second common electrode 7 b areelectrically connected to each other through a contact hole 22 definedin the insulating layer 12*.

[0087] The common electrodes 7 have a higher potential than the gatesignal lines 4 for most of a driving period. By forming apertures 13 cin the parts of the overcoat film 12 where a common electrode 7 iscovered with the overcoat film 12, the ion retrieval function can beimparted to the parts of the gate signal lines 4 exposed through theapertures 13 c.

[0088] For direct exposure of the common electrodes of the arraysubstrate used in the liquid crystal screen displays of the first andother embodiments for instance, it is necessary to form an aperturewhich penetrates through the insulating layer and the overcoat filmbecause the insulating layer and the overcoat film are located upperthan the common electrodes. As the depth of the aperture increases, itbecomes difficult to smoothly retrieve ions within the liquid crystallayer. In addition, the process for forming the hole that penetratesthrough a plurality of layers is troublesome.

[0089] In contrast with this, the fifth embodiment makes it possible toform the second common electrode 7 b, which serves as a common electrodeelement having an aperture just above it, in a layer upper than thefirst common electrode 7 a so as to be in contact with the overcoat film12, so that the above-described obstacle to ion retrieval can beeliminated. In addition, electrical short between each first commonelectrode 7 a and gate signal line 4 can be prevented in the process ofintegral formation of them. Even though each common electrode 7 iscomposed of two kinds of elements, that is, the first common electrode 7a and the second common electrode 7 b, its function for establishing anelectric field between the pixel electrodes 6 and it is not impaired.

[0090] The first common electrode 7 a is formed through a processsimilar to the processes of the foregoing embodiments. The contact hole22 is formed at the same time that the insulating layer 11 at the end ofthe substrate 10 is removed to expose the terminal used for connectingthe gate signal lines 4 to the external circuit. The second commonelectrode 7 a* is formed simultaneously with the formation of the sourcesignal lines 5, the pixel electrodes 6 and others. Accordingly, theliquid crystal screen display of the fifth embodiment can beaccomplished without adding a new step to the fabrication process of theconventional liquid crystal screen display.

[0091] If all the common electrodes 7 are respectively formed from twokinds of elements, i.e., the first common electrode 7 a and the secondcommon electrode 7 b as shown in FIGS. 12, 13a and 13 b, electricalshort between each first common electrode 7 a and gate signal line 4 canbe more reliably prevented in the process of integral formation of themand further, more effective ion-retrieval is ensured when the displaypanel is in its driven state.

[0092] It is desired for more effective ion retrieval that the potentialof the common electrodes be constant during driving of the panel.

Six Embodiment

[0093] The six embodiment will be explained in conjunction with anotherliquid crystal screen display capable of performing effective iongeneration and ion retrieval.

[0094]FIG. 14 shows an array substrate for use in the liquid crystalscreen display of the six embodiment. The array substrate 2 has commonelectrodes one of which is composed of a plurality of elements placed ondifferent layers like the fifth embodiment and the overcoat film 12 isformed on the TFT 9 and its peripheral region alone. Therefore, thesource signal lines 5, the pixel electrodes 6 and the second commonelectrode 7 b are exposed.

[0095] These exposed elements have the ion retrieval function.Accordingly, ions can be retrieved in a large area.

[0096] The array substrate of the six embodiment can be fabricated inthe same way as the seventh embodiment except the processing pattern forthe overcoat film 12.

[0097] The liquid crystal screen display of the sixth embodiment hasproved successful in displaying satisfactory, unevenness-free imagesafter continuous driving at 50° C. for 1000 hours.

[0098] The first to six embodiments have been presented on the conditionthat the ionized liquid crystal molecules within the liquid crystallayer 18 are anions. This is based on such findings attained by thepresent inventors after conducting many experiments that one of thecauses of display unevenness occurring when images are displayed on thescreen with the gate signal lines 4 exposed by laser irradiation isanionized liquid crystal molecules. Therefore, in a liquid crystalscreen display having cationized liquid crystal molecules in the liquidcrystal layer, the positive/negative and high/low relationships ofvoltage are all reversed. Accordingly, the voltage applied to theconductive member such as the gate signal lines or the pixel electrodesis not “a negative voltage” but “a positive voltage”. Likewise, thevoltage applied to the second conductive member is not “a positivevoltage” but “a negative voltage”.

Seventh Embodiment

[0099] The seventh embodiment will be described, taking a case forexample in which ion generation and ion retrieval are more effectivelycarried out.

[0100]FIG. 15 shows an array substrate for use in the liquid crystalscreen display of the seventh embodiment. The array display 2 has theovercoat film apertures 13 a in the regions where the gate signals 4 areformed similarly to the third embodiment and the overcoat film aperture13 b immediately above the common electrode line 8. The array substrate2 further comprises catalyst layers 19 a, 19 b that are adjacent to theapertures 13 a, 13 b and include fine titanium oxide particles as acatalyst. According to the liquid crystal screen display of the seventhembodiment, in the regions where the ion generation apertures 13 a andthe ion retrieval aperture 13 b are defined by the catalyst layers 19 aand 19 b, activation energy caused by the reaction between ionizedliquid crystal molecules and unionized liquid crystal molecules arelowered to speed up the electrochemical reaction between them, wherebymore effective ion generation and ion retrieval are carried out.

[0101] The catalyst layers 19 a and 19 b are formed in such a way that,for example, a paste of photo-curable resin prepolymer having finecatalyst particles dispersed therein is applied to the surface of thesubstrate 10 where the overcoat film 12 is formed and then, the coatedfilm thus formed is processed into a desired shape. The catalyst layers19 a, 19 b are cured and shrunk by heating so that clacks are created ontheir surfaces to increase their surface area, thereby obtaining moresignificant catalytic effects.

[0102] Examples of the catalyst for the catalyst layers includeinorganic semiconductor catalysts such as tin oxide; precious metalcatalysts such as platinum, palladium and rubidium; and organicsemiconductor catalysts such as polyaniline and polythiophene. Thecatalyst layers prepared by fine particles carrying a catalyst have alarge surface area and therefore exert significant catalytic effects.For effective ion generation and ion retrieval, the catalysts layers arepreferably positioned in the vicinity of the conductive member for iongeneration and the conductive member for ion retrieval.

Eighth Embodiment

[0103] The eighth embodiment will be described, taking a case forexample in which an ion absorbable substance is used for ion retrieval.

[0104] A paste prepared by dispersing fine particles of aluminum oxidecarrying nickel as a catalyst in a photo-curable resin is applied to theblack matrix 14, and the coated film thus formed is processed to form anion absorbing layer 21 as shown in FIG. 16. The placement of the ionabsorbable substance in contact with the liquid crystal layer obviatesthe need for processing the overcoat film etc. in order to form an ionretrieval element like the foregoing embodiments.

Ninth Embodiment

[0105] Display unevenness due to uneven ion distribution can beeffectively restrained by dissolving an ionic substance, that is,electrolytic salt in the liquid crystal layer beforehand. Even if ionsare generated by the potential of the gate signal lines and otherswithin the liquid crystal layer, variations in ion concentration areinsignificant so that occurrence of display unevenness can besuppressed. For instance, (t-Bu)₄NX (X is halogen) such as representedby (t-Bu)₄NCl and (t-Bu)₄NBr; (t-Bu)₄NCOOR (R═H, a hydro carbon groupsuch as C_(2n)H_(2n+1), or alkali metal) or the like is added in anamount of 200 ppm to 1000 ppm.

Industrial Applicability

[0106] According to the present invention, there are provided liquidcrystal screen displays capable of restraining occurrence of displayunevenness attributable to ions generated within the liquid crystallayer and displaying satisfactory images after driving for a long time.Therefore, the invention highly contributes to improvements in theperformance and reliability of liquid crystal displays.

What is claimed is:
 1. A liquid crystal screen display comprising: afirst insulating substrate; a second insulating substrate facing thefirst insulating substrate; a liquid crystal layer formed between thefirst and second insulating substrates; alignment layers formed betweenthe first insulating substrate and the liquid crystal layer and betweenthe second insulating substrate and the liquid crystal layer,respectively, for aligning the liquid crystal layer; and a firstconductive member which is formed between at least either one of thefirst and second insulating substrates and its corresponding alignmentlayer, being in partial contact with the alignment layer and to which anegative voltage is applied.
 2. The liquid crystal screen displayaccording to claim 1, wherein the first conductive member is formed onthe first insulating substrate.
 3. The liquid crystal screen displayaccording to claim 2, wherein the first conductive member is gate signallines.
 4. The liquid crystal screen display according to claim 1,wherein regions accounting for 10 to 50% of the first conductive memberare in contact with the alignment layer.
 5. The liquid crystal screendisplay according to claim 3, wherein an oxidation layer is formed inthe regions of the gate signal lines where the gate signal linesrespectively intersect source signal lines.
 6. The liquid crystal screendisplay according to claim 1, wherein the first conductive member isformed on the second insulating substrate.
 7. The liquid crystal screendisplay according to claim 6, wherein pixel electrodes and counterelectrodes for applying a voltage to a liquid crystal of the liquidcrystal layer are formed on the first insulating substrate.
 8. Theliquid crystal screen display according to claim 1, further comprising alight blocking layer formed on the first or second insulating substrate,wherein the first conductive member is formed so as to overlap with thelight blocking layer in a plan view of the display.
 9. The liquidcrystal screen display according to claim 1, further comprising a secondconductive member which is disposed between at least either one of thefirst and second insulating substrates and its corresponding alignmentlayer, being in partial contact with the alignment layer, and to which avoltage higher than the negative voltage applied to the first conductivemember is applied.
 10. The liquid crystal screen display according toclaim 9, wherein the second conductive member is formed on the firstinsulating substrate.
 11. The liquid crystal screen display according toclaim 10, further comprising: a switching element formed on the firstinsulating substrate, the switching element having a source electrodeconnected to the source signal lines, a gate electrode connected to thegate signal lines and a drain electrode connected to the pixelelectrodes; and counter electrodes formed on the first insulatingsubstrate for applying a voltage to a liquid crystal of the liquidcrystal layer between the counter electrodes and the pixel electrodes;wherein the second conductive member is the pixel electrodes.
 12. Theliquid crystal screen display according to claim 10, further comprising:a switching element formed on the first insulating substrate, theswitching element having a source electrode connected to the sourcesignal lines, a gate electrode connected to the gate signal lines and adrain electrode connected to the pixel electrodes; and counterelectrodes formed on the first insulating substrate for applying avoltage to a liquid crystal of the liquid crystal layer between thecounter electrodes and the pixel electrodes; wherein the secondconductive member is the counter electrodes.
 13. The liquid crystalscreen display according to claim 10, further comprising: a switchingelement formed on the first insulating substrate, the switching elementhaving a source electrode connected to the source signal lines, a gateelectrode connected to the gate signal lines and a drain electrodeconnected to the pixel electrodes; and counter electrodes formed on thefirst insulating substrate for applying a voltage to a liquid crystal ofthe liquid crystal layer between the counter electrodes and the pixelelectrodes; wherein the second conductive member is the pixel electrodesand the counter electrodes.
 14. The liquid crystal screen displayaccording to claim 10, further comprising: a switching element formed onthe first insulating substrate, the switching element having a sourceelectrode connected to the source signal lines, a gate electrodeconnected to the gate signal lines and a drain electrode connected tothe pixel electrodes; and counter electrodes formed on the secondinsulating substrate for applying a voltage to a liquid crystal of theliquid crystal layer between the counter electrodes and the pixelelectrodes; wherein the second conductive member is the pixelelectrodes.
 15. The liquid crystal screen display according to claim 9,wherein the second conductive member is formed on the second insulatingsubstrate.
 16. The liquid crystal screen display according to claim 15,further comprising: a switching element formed on the first insulatingsubstrate, the switching element having a source electrode connected tothe source signal lines, a gate electrode connected to the gate signallines and a drain electrode connected to the pixel electrodes; andcounter electrodes formed on the first insulating substrate for applyinga voltage to a liquid crystal of the liquid crystal layer between thecounter electrodes and the pixel electrodes.
 17. The liquid crystalscreen display according to claim 15, further comprising: a switchingelement formed on the first insulating substrate, the switching elementhaving a source electrode connected to the source signal lines, a gateelectrode connected to the gate signal lines and a drain electrodeconnected to the pixel electrodes; and counter electrodes formed on thesecond insulating substrate for applying a voltage to a liquid crystalof the liquid crystal layer between the counter electrodes and the pixelelectrodes; wherein the second conductive member is the counterelectrodes.
 18. The liquid crystal screen display according to claim 9,further comprising a light blocking layer formed on the first or secondinsulating substrate, wherein the second conductive member is formed soas to overlap with the light blocking layer in a plan view of thedisplay.
 19. The liquid crystal screen display according to claim 1,further comprising a catalyst placed in the vicinity of the firstconductive member in contact with its corresponding alignment layer, forreducing activation energy caused by the reaction between unionizedliquid crystal molecules and ionized liquid crystal molecules.
 20. Theliquid crystal screen display according to claim 19, wherein thecatalyst includes a metal oxide.
 21. The liquid crystal screen displayaccording to claim 19, wherein the catalyst includes a precious metal.22. The liquid crystal screen display according to claim 19, wherein thecatalyst includes an organic semiconductor compound.
 23. The liquidcrystal screen display according to claim 1, further comprising an ionabsorber provided for the second insulating substrate.
 24. The liquidcrystal screen display according to claim 23, further comprising: aswitching element formed on the first insulating substrate, theswitching element having a source electrode connected to the sourcesignal lines, a gate electrode connected to the gate signal lines and adrain electrode connected to the pixel electrodes; and counterelectrodes formed on the first insulating substrate for applying avoltage to a liquid crystal of the liquid crystal layer between thecounter electrodes and the pixel electrodes.
 25. A liquid crystal screendisplay comprising: a first insulating substrate; a second insulatingsubstrate facing the first insulating substrate; a liquid crystal layerformed between the first and second insulating substrates; and anelectrolyte added to the liquid crystal layer.
 26. The liquid crystalscreen display according to claim 25, wherein the electrolyte is acompound given by the chemical formula, (t-Bu)₄NX.
 27. The liquidcrystal screen display according to claim 26, wherein X of the chemicalformula is halogen.
 28. The liquid crystal screen display according toclaim 27, wherein X of the chemical formula is COOR (in which R ishydrogen, a hydro carbon group or alkali metal).